It is desirable in some types of keyboards to incorporate a key return mechanism which provides downstroke resistance which decreases as the key is depressed. An example of such a keyboard is one incorporating a tactile orientation system, e.g., Braille. Another tactile orientation system is described in the present applicant's U.S. Pat. application for "Tactile Key Tops," Ser. No. 173,855, filed Dec. 22, 1993. The key return means must provide sufficient initial downstroke resistance to allow the operator to feel a tactile feature on a key top without depressing the key. At the same time, however, it is generally desirable to minimize the overall energy, i.e., force through distance, which is required to fully depress the key. Minimizing this overall required energy reduces finger fatigue. To provide optimum initial downstroke resistance while minimizing finger fatigue, downstroke resistance which decreases during key depression is thus required.
Also, in some applications, e.g., musical keyboards, the operator is often required to hold one or more keys in depressed position for a period of time. If each key provides a low upward force when fully depressed, finger fatigue will be further reduced in these applications.
Other advantages of decreasing downstroke resistance are discussed in various prior art including U.S. Pat. Nos. 3,478,857 (Linker) and 4,476,769 (Kumano).
Various key return means have been developed in the prior art which provide decreasing downstroke resistance. One such means utilizes a swing arm, or rotor, which serves to transmit return force from a key return element (such as a coil spring, leaf spring, or weight) to a key. The swing arm/key combination may be geometrically configured to provide decreasing downstroke resistance to the striking surface of the key.
In all examples of the known prior art which apply swing arms to musical keyboards, the swing arms are disposed to rotate on a longitudinal axis, i.e., an axis parallel to the length of the keyboard. This axis disposition has been found in the prior art to necessitate swing arm mounting structures which are complex, bulky and costly.
Swing arms can also be found in self-contained switches of the type used in computer keyboards. Examples are shown in U.S. Pat. Nos. 4,249,055 (Matsuo) and 4,803,316 (Hayashi). These switches are relatively compact, but are formed of a large number of parts and thus are expensive to manufacture. In each of these switches, a key position sensing element is operated directly by the swing arm. This aids in forming a compact mechanism. However, the key position sensing element in each switch has its own miniature circuit board. This circuit board is in addition to the master circuit board which the switch is mounted on. Other structures, such as the switch housing, add manufacturing cost as well. Furthermore, the swing arm of one of these switches is very small in size and thus cannot accommodate a large key dip directly. A coil spring may be used as a link between the plunger and swing arm as in the two patents mentioned in this paragraph above. This spring allows for increased key dip, but introduces an initially increasing return force on the key. This increasing return force is sometimes undesirable as set forth above. For these and other reasons switches of this type are not well suited for musical applications.
In applying a swing arm mechanism to a Janko Keyboard with independent keys (as described in the grandparent application of this specification) one finds that the above mentioned drawbacks of the prior art take on increased significance. Space underneath the keyboard is at a premium. There is little room for the support structure, or frame, and necessary mechanisms such as key return means, key motion sensing means, bushings, guides, and limit-of-travel stopping means. With slidably interlocking chassis rails supporting the key rows as described in the grandparent application and also described below, swing arms with longitudinal axes would have to be very short or would have to pass through ports in the rails, which would increase manufacturing costs. Furthermore, with 2.5 times the standard number of keys (assuming a 5-row design) the cost of self-contained switches becomes quite high.